Channel quality index feedback reduction for broadband systems

ABSTRACT

A system and methodology for exploiting channel correlation in time and/or frequency to reduce CQI feedback in wireless communications systems. By compressing CQI feedback at the receiver to reduce redundancy in CQI feedback information that results from the channel correlation, the average feedback rate is reduced. Redundancy in time may be removed from the CQI feedback by monitoring variations of the CQI information in time at the receiver so that a codebook index for a given reporting instance is communicated to the transmitter only if it differs from the codebook index for the previous reporting instance. Otherwise, no feedback is performed.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention is directed in general to field of information processing. In one aspect, the present invention relates to a system and method for reducing channel quality indicator (CQI) feedback in communication systems.

Description of the Related Art

Wireless communication systems transmit and receive signals within a designated electromagnetic frequency spectrum, but the capacity of the electromagnetic frequency spectrum is limited. As the demand for wireless communication systems continues to expand, there are increasing challenges to improve spectrum usage efficiency. To improve the communication capacity of the systems while reducing the sensitivity of the systems to noise and interference and limiting the power of the transmissions, a number of wireless communication techniques have been proposed.

FIG. 1 depicts a wireless communication system 100 in which a transmitter 102 having a single antenna or an array of antennas 106 communicates with receiver 104 having a single antenna or an array of antennas 108. The communication system 100 may be any type of wireless communication system, including but not limited to a MIMO system, SDMA system, CDMA system, OFDMA system, OFDM system, etc. In the communication system 100, the transmitter 102 may act as a base station, while the receiver 104 acts as a subscriber station, which can be virtually any type of wireless one-way or two-way communication device such as a cellular telephone, wireless equipped computer system, and wireless personal digital assistant. Of course, the receiver/subscriber station 104 can also transmits signals which are received by the transmitter/base station 102. The signals communicated between transmitter 102 and receiver 104 can include voice, data, electronic mail, video, and other data, voice, and video signals. In operation, the transmitter 102 transmits a signal data stream (e.g., signal s₁) through one or more antennas 106 and over a channel H₁ to a receiver 104, which combines the received signal from one or more receive antennas 108 to reconstruct the transmitted data. To transmit the signal vector s₁, the transmitter 102 prepares a transmission signal, represented by the vector x₁, for the signal s₁. (Note: lower case bold variables indicate vectors and upper case BOLD variables indicate matrices). The transmission signal vector x₁ is transmitted via a channel represented by a channel matrix H₁, and is received at the receiver 104 as a receive signal vector y₁=H₁x₁+n₁ (where n represents co-channel interference or noise). The channel matrix H₁ represents a channel gain between the transmitter antenna array 106 and the subscriber station antenna array 108. Thus, the channel matrix H₁ can be represented by an k×N matrix of complex coefficients, where N is the number of antennas in the transmitter/base station antenna array 106 and k is the number of antennas in the receiver/subscriber station antenna array 108. The value of k can be unique for each receiver/subscriber station. As will be appreciated, the channel matrix H₁ can instead be represented by a N×k matrix of complex coefficients, in which case the matrix manipulation algorithms are adjusted accordingly so that, for example, the right singular vector calculation on a k×N channel matrix becomes a left singular vector calculation on a N×k channel matrix. The coefficients of the channel matrix H₁ depend, at least in part, on the transmission characteristics of the medium, such as air, through which a signal is transmitted. A variety of methods may be used at the receiver to determine the channel matrix H₁ coefficients, such as transmitting a known pilot signal to a receiver so that the receiver, knowing the pilot signal, can estimate the coefficients of the channel matrix H₁ using well-known pilot estimation techniques. Alternatively, when the channel between the transmitter and receiver are reciprocal in both directions, the actual channel matrix H₁ is known to the receiver and may also be known to the transmitter.

In broadband communication systems which use multi-carrier modulation systems such as ODFM/OFDMA, channel quality indicators (CQI) such as “carrier to interference plus noise” ratio (CINR) are fed back from the receiver to the transmitter, in order to select the appropriate transmission profile. For instance, adapting the modulation and coding rates, and scheduling multiple users based on CQI improves data rates and link reliability. This technology has been, or will be, adopted in many current and future standards such as IEEE 802.16, 3GPP LTE, etc.

Feeding back the CQI for multi-carrier systems such as OFDMA can potentially use up the usually small bandwidth allocated to the feedback control channel due to the multiplicity of the CQI coefficients. Accordingly, there is a need for a system and method for reducing the CQI feedback overhead is these systems. Further limitations and disadvantages of conventional processes and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be understood, and its numerous objects, features and advantages obtained, when the following detailed description of a preferred embodiment is considered in conjunction with the following drawings, in which:

FIG. 1 (labeled prior art) depicts a wireless communication system.

FIG. 2a depicts a wireless communication system in which CQI feedback is compressed in time and frequency at a receiver prior to feed back to a transmitter.

FIG. 2b depicts a generalized system flow for temporally compressing CQI feedback for broadband channels.

FIG. 3 depicts an example flow for a first methodology for using discrete cosine transform techniques to generate and feed back CQI data that is compressed in time and frequency.

FIG. 4a depicts a wireless communication system in which CQI feedback is compressed in time and frequency by using discrete cosine transform techniques at a receiver prior to feed back to a transmitter.

FIG. 4b depicts a generalized system flow for using discrete cosine transforms for temporally compressing CQI feedback for broadband channels.

FIG. 5 depicts an example flow for a second methodology for generating and feeding back a CQI data that is compressed in time.

It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for purposes of promoting and improving clarity and understanding. Further, where considered appropriate, reference numerals have been repeated among the drawings to represent corresponding or analogous elements.

DETAILED DESCRIPTION

A system and methodology are disclosed for exploiting channel correlation in time and/or frequency to reduce CQI feedback in wireless communication systems. The embodiments of the invention disclosed herein can be implemented in single antenna communication systems or in communication systems wherein the transmitter and/or the receiver comprise multiple antennas. By compressing CQI feedback at the receiver to reduce redundancy in CQI feedback information that results from channel correlation, the average feedback rate is reduced. In some embodiments, redundancy in time may be removed from the CQI feedback information by monitoring variations of the CQI information in time at the receiver so that CQI information for a given CQI reporting instance is communicated to the transmitter only if it differs from the CQI information for the previous CQI reporting instance. Otherwise, no feedback is performed. Compression of the CQI feedback information in time and frequency exploits the temporal correlation of the transmission channel wherein, statistically, the characteristics of the transmission channel change slowly over time in most cases of interest. This compression technique may also be applied to broadband communication systems by dividing a broadband channel into a plurality of clusters (where each cluster consists of one or more sub-carriers in a given frequency range) and then sending back CQI information only for those clusters where there is a change in CQI information, along with bit indices of the corresponding clusters to indicate cluster numbers. Additional compression may be obtained by limiting the number clusters for which temporally compressed CQI information is fed back, such as by ranking the candidate clusters according to a given predetermined or dynamic selection criterion and sending back the temporally compressed CQI information only for a predetermined number of the top-ranked clusters, where the predetermined number may be fixed or may be adjusted dynamically for different feedback intervals. In one embodiment, the given selection criterion for ranking the candidate cluster may be the absolute values such that the candidate clusters are ranked from highest CQI value to lowest CQI value, so that the top ranked candidate clusters correspond to the ones have highest CQI values. In another embodiment, the candidate clusters are ranked according to the changes in their CQI values from one reporting period to the next reporting period, so that the top ranked candidate clusters correspond to those clusters whose CQI values have changed the most. In some embodiments, CQI feedback is compressed by performing a multiple-input, multiple-output transform, such as the discrete cosine transform (DCT), on the CQI data, thereby generating a plurality of transform coefficients. In these embodiments, the transform coefficients are evaluated during a reporting instance, ‘n’, by comparing these coefficients to corresponding coefficients in a reporting instance, ‘n−1’. In embodiments of the invention, the compressed feedback comprises all transform coefficients that have changed during a particular interval. In another embodiment, the feedback information comprises only the changed coefficients that meet a predetermined metric. This predetermined metric in one embodiment can be a threshold, where the threshold may be different for each DCT coefficient. In other embodiments, the candidate DCT coefficients can be selected in accordance with a dynamic metric which, in some embodiments, can be a dynamic threshold, In each of the embodiments described herein, the compressed CQI feedback information is communicated to the transmitter through the feedback control channel where it is processed to regenerate the original CQI state information and is used for scheduling and automatic modulation control (AMC). As used herein, CQI information is interpreted to include CQI state information comprising the actual CQI values, index information that can be used to represent CQI values, and/or CQI information obtained by performing a transform of CQI values, such as the multiple-input, multiple output transforms described hereinbelow.

Various illustrative embodiments of the present invention will now be described in detail with reference to the accompanying figures. While various details are set forth in the following description, it will be appreciated that the present invention may be practiced without these specific details, and that numerous implementation-specific decisions may be made to the invention described herein to achieve the device designer's specific goals, such as compliance with process technology or design-related constraints, which will vary from one implementation to another. While such a development effort might be complex and time-consuming, it would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. For example, selected aspects are shown in block diagram form, rather than in detail, in order to avoid limiting or obscuring the present invention. In addition, some portions of the detailed descriptions provided herein are presented in terms of algorithms or operations on data within a computer memory. Such descriptions and representations are used by those skilled in the art to describe and convey the substance of their work to others skilled in the art.

FIG. 2a depicts a wireless communication system 200 in which a transmitter 202 uses CQI feedback techniques to configure or adapt one or more input signals that are transmitted from a transmitter 202 (e.g., a base station) to one or more receivers 206.1-206.m (e.g., subscriber stations), where the CQI feedback values are compressed in time and/or frequency at a receiver 206.i prior to feed back to a transmitter 202. Those of skill in the art will understand that each receiver 206.i will provide a complete set of CQI information when it is initialized. Thereafter, the techniques described in connection with various embodiments of the invention can be used on reduce the amount of CQI information fed back to the transmitter 202.

The transmitter 202 includes a single antenna or an array or antennas 227 for communicating with the receivers 206.1-m. The receivers 206.1-m, may comprise a single antenna or an array of antennas 209.i for communicating with the transmitter 202. In operation, a data signal s_(i) presented at the transmitter 202 for transmission to the receiver 206.i is transformed by the signal processor 221.i into a transmission signal represented by the vector x_(i). The signals transmitted from the transmit antenna 227 propagate through a matrix channel H_(i) and are received by the receive antennas 209.i where they are represented by the vector y_(i). For a channel from the transmitter 202 to the i^(th) receiver 206.i, the channel is denoted by H_(i), iε{1, 2, . . . , m}. The channel matrix H_(i) may be represented as an k_(i)×N matrix of complex entries representing the complex coefficients of the transmission channel between each transmit-receive antenna pair, where N represents the number of antennas of the transmitter 202 and k_(i) represents the number of antennas of the i^(th) receiver 206.i. At the receiver 206.i, the signal processing unit 205.i processes the y_(i) signals received on the k antennas to obtain a data signal, z_(i), which is an estimate of the transmitted data s_(i). The processing of the received y_(i) signals may include combining the y_(i) signals with appropriate combining vector information retrieved from the codebook 207.i.

The channel matrix H_(i)—which specifies the transmission channel between a transmitter and an i^(th) receiver—in the channel estimation signal processing unit 205.i. For example, in a MIMO implementation, each receiver 206.1-m determines its MIMO channel matrix H_(i) by using pilot estimation or sounding techniques to determine or estimate the coefficients of the channel matrix H_(i). Each receiver 206.i uses the estimated MIMO channel matrix or other channel-related information (which can be channel coefficients or channel statistics or their functions, such as a precoder, a beamforming vector or a modulation order) to generate CQI information that is to be used to configure/adapt the signals transmitted by the transmitter.

Rather than feeding back the full CQI representation, the receiver 206.i may use a codebook 207.i to compress or quantize the transmission profile (e.g., CQI information) that is generated from the detected channel information and that can be used by the transmitter in controlling signal transmission to the receiver. The CQI estimator 203.i generates a quantization/codebook index by accessing the receiver codebook 207.i which stores an indexed set of possible transmission profiles and/or channel matrices along with associated CQI information so that the estimated channel matrix information 204.i generated by the signal processing unit 205.i can be used by the CQI estimator 203.i to retrieve a codebook index from the codebook 207.i. The output of the CQI estimator 203.i is provided to a CQI transition detector 212.i that is operable to detect changes in the CQI information. If the CQI transition detector 212.i detects a change in the CQI information of any cluster, the new CQI information is communicated to the transition subset selector 214.i for transmission to the transmitter 202 via the feedback channel 215, in accordance with selection metrics described in greater detail below.

A selected subset of the CQI information, which may be in the form of indexed information, is transmitted via the feedback channel 215 and is stored in memory 220 of the transmitter 202. A memory controller 222 is operable to control the updating of the indexed CQI information in the memory 220 and to control the transfer of the indexed CQI information to the decoder 224. The decoder 224 is operable to process the indexed CQI information and to retrieve information from the codebook 226 to provide CQI information that can be used by scheduling module 225 and AMC selection module 228 to generate scheduling or AMC information, respectively, for a particular receiver 206.i.

The CQI information communicated to the transmitter 202 via the feedback channel 215 can be provided in a plurality of alternative formats. In one embodiment of the invention, the feedback subset comprises all CQIs that have changed during a particular CQI reporting instance. In this embodiment, the feedback subset may be limited to changed CQI information only, or it may include changed CQI information and a predetermined quantity of unchanged CQI information. In another embodiment, the feedback information transmitted to the transmitter 202 comprises a subset of only the changed CQIs that meet a predetermined selection criterion. In one embodiment, clusters corresponding to the changed CQI are ranked from highest CQI value to lowest CQI value, so that the top ranked ‘F’ candidate clusters (where ‘F’ is some integer number) corresponding to the ones have highest CQI values are chosen to be fed back. In another embodiment, the candidate clusters are ranked according to the changes in their CQI values from one reporting instance to the next reporting instance, so that the top ranked candidate clusters corresponding to those clusters whose CQI values have changed the most are fed back. In yet another embodiment, the candidate CQIs are selected in accordance with a dynamic selection criterion. In one embodiment, the dynamic selection criterion is such that the number ‘F’, of top ranked clusters whose CQI value are fed back, itself changes from one reporting instance to another.

The CQI feedback compression techniques described above may be extended and applied to a variety of communication systems, including but not limited to TDD systems, FDD systems, OFDM systems and broadband systems. For example, FIG. 2b depicts a generalized system flow for temporally compressing CQI feedback for a broadband channel. The channel may be divided into a plurality of clusters, where each cluster consists of one or more sub-carriers in a given frequency range. In FIG. 2b , the clusters are depicted as a sequence of contiguous and localized sub-carrier clusters 240 (Cluster 1, Cluster 1, . . . Cluster N), though there is no requirement that the clusters necessarily be either contiguous or localized. For example, a set of non-continuous sub-carriers can be mapped to each cluster. Likewise, though each cluster may contain a predetermined number of M sub-carriers, there is no requirement that each cluster have the same number of sub-carriers.

A single processor or controller could be used to detect channel transitions for all the clusters or for a subset of the clusters. Alternatively, and as illustrated in FIG. 2b , N separate channel transition detection circuits could be used, so that a first channel transition detector 242 is used to detect changes in the CQI feedback from a first cluster (or group of clusters), a second channel transition detector 244 is used to detect changes in the CQI feedback from a second cluster (or group of clusters), and so on. By sending CQI index information over the feedback channel 248 to the transmitter or base station only for those clusters whose CQI has changed in time, the amount of feedback may be significantly reduced, thereby providing efficient CQI feedback for the broadband channel.

Notwithstanding the efficiency benefits of using temporal feedback compression, a significant amount of feedback overhead may be consumed when feeding back CQI information for a broadband channel. Accordingly, a selected embodiment of the present invention further reduces the feedback overhead by sending temporally compressed feedback for only a subset of the clusters that have changed. In effect, an upper bound is placed on the number of clusters for which CQI feedback is allowed. For example, rather than sending feedback for all the clusters that have changed, feedback is sent only for a number F (where 1≦F<N) of clusters from the clusters that have changed. Again, the feedback will consist of the changed CQI indices and log 2(N) bits indicating which cluster has changed. To select which of the clusters having changed CQIs will provide feedback, the clusters can be ranked or sorted or otherwise chosen with any desired technique. For example, the candidate clusters may be ranked by their corresponding channel quality indicator, such as a CINR (Carrier-to-Interference plus Noise Ratio) or SINR (Signal-to-Interference plus Noise Ratio) value. Once the candidate clusters are ranked, F of the top-ranked candidate clusters are chosen for feedback. As will be appreciated, the fixed number F may be fixed across all feedback intervals, or may be variable for each feedback interval. When F is fixed, then the size of feedback is fixed, and when F is variable, the size of feedback is also variable. When F is fixed, and if less than F clusters, say C clusters, have changed CQI indices, then it is to be understood that zeros are sent in place of the F-C clusters or alternate information can sent in place of the F-C clusters.

FIG. 3 is a flowchart illustration of the processing flow for implementing the embodiments of the invention described above in connection with FIGS. 2a-b . In step 302, processing is initiated, followed by the designation, in step 304, of a set of clusters to be monitored for CQI changes. In step 306, the CQI is estimated for each of the designated clusters during a CQI reporting instance, ‘n’. In step 308, CQI data for the changed clusters is compiled. In step, 310 a subset of the CQI data for the changed clusters is chosen in accordance with predetermined selection metrics. This data is then compiled in step 312 and transmitted from the receiver to the transmitter over a feedback channel in step 314. In step 316, the transmitter uses the CQI data for scheduling and AMC. Processing then proceeds to step 318 where the CQI reporting instance is incremented and the steps discussed above are repeated.

FIG. 4a depicts a wireless communication system 400 wherein discrete cosine transformation techniques are used to compress CQI feedback values at a receiver 206.i prior to feed back to a transmitter 202. Many of the components of the transmitter 202 and the receiver 206 i are substantially the same as discussed above in connection with FIG. 2a and, therefore, the discussion of these components will not be repeated. In the communication system shown in FIG. 4a , the CQI estimator 203.i generates a CQI value that is processed by a discrete cosine transform 412.i to generate a plurality of DCT coefficients. These DCT coefficients are provided to a DCT transition detector 414.i that is operable to detect a change in the coefficients that indicates a change in the DCT (and hence CQI) values. The output of the DCT transition detector 414.i is provided to a DCT coefficient selector 416.i that is operable to transmit DCT coefficient information to the transmitter 202, in accordance with predetermined selection metrics/criteria, for example a threshold-based criterion.

The DCT coefficient information, which may be in the form of indexed information, is transmitted via the feedback channel 215 and is received by the transmitter 202 and stored in memory 220. A memory controller 222 is operable to control the updating of the indexed DCT coefficients in the memory 220 and to control the transfer of the indexed DCT coefficients to the inverse DCT transform module 418. The inverse DCT module 418 then performs an inverse DCT transform on the updated DCT coefficients for each cluster to regenerate CQI index information for each cluster. The CQI index information is provided to a decoder 224 that is operable to retrieve information from the codebook 226 for CQI information for a particular receiver 206.i. The CQI information then used by the transmitter 202 for scheduling and AMC operations. The DCT coefficient information communicated to the transmitter 202 via the feedback channel 215 can be provided in a plurality of alternative formats. In one embodiment of the invention, the feedback comprises all DCT coefficients that have changed during a particular interval. In another embodiment, the feedback information transmitted to the transmitter 202 comprises only the changed DCT coefficients that meet a predetermined metric. This predetermined metric in one embodiment can be a threshold, i.e. a given DCT coefficient is chosen to be fed back if the change in its value from one reporting instance to another exceeds a certain predetermined threshold, where the threshold may be different for each DCT coefficient. In yet another embodiment, the candidate DCT coefficients are selected in accordance with a dynamic metric; this dynamic metric in one embodiment can be a dynamic threshold, i.e. a given DCT coefficient is chosen to be fed back if the change in its value from one reporting instance to another exceeds a certain threshold, where the threshold may be different for each DCT coefficient and may change from one reporting instance to another.

FIG. 4b depicts a generalized system flow for temporally compressing CQI feedback for a broadband channel using DCT techniques. To implement DCT-based CQI compression for the broadband channel, the channel may be divided into a plurality of clusters, where each cluster consists of one or more sub-carriers in a given frequency range. In FIG. 4b , the clusters are depicted as a sequence of contiguous and localized sub-carrier clusters 440 (Cluster 1, Cluster 1, . . . Cluster N), though there is no requirement that the clusters necessarily be either contiguous or localized. For example, a set of non-continuous sub-carriers can be mapped to each cluster. Likewise, though each cluster may contain a predetermined number of M sub-carriers, there is no requirement that each cluster have the same number of sub-carriers.

As will be understood by those of skill in the art, the discrete cosine transform is a multiple-input, multiple-output transform. The discrete cosine transform of CQI information for ‘N’ clusters will generate ‘N’ DCT coefficients. Furthermore, those of skill in the art will recognize that the embodiments of the invention described herein can be implemented using other multiple-input, multiple output transforms to process the CQI information for the plurality of clusters. For example, embodiments of the present invention can be implemented using a fast-Fourier transform.

As illustrated in FIG. 4b , N separate CQI estimator circuits can be used, so that a first CQI estimator 442 is used to detect changes in the CQI feedback from a first cluster (or group of clusters), a second CQI estimator 444 is used to detect changes in the CQI feedback from a second cluster (or group of clusters), and so forth. A discrete cosine transform 448 is performed across the ‘N’ cluster CQI outputs of the CQI estimators 442, 444, and 446 to generate DCT coefficients corresponding to the CQI changes in each of the clusters. The DCT coefficients are processed by DCT coefficient transition detectors 450, 452 and 454 to detect changes in the DCT coefficients. The DCT coefficient selector in the receiver then selects the best ‘M’ coefficients, in accordance with predetermined selection metrics, and transmits the corresponding cluster numbers and DCT indices to the transmitter via the feedback channel.

FIG. 5 is a flow chart illustration of the processing flow for implementing the embodiments of the invention described above in connection with FIGS. 4a-b . In step 502, processing is initiated, followed by the designation, in step 504, of a set of clusters to be monitored for CQI changes. In step 506, the CQI is estimated for each of the designated clusters during a CQI reporting instance, ‘n’. In step 508, a DCT transform is performed across the ‘N’ clusters to obtain a plurality of DCT coefficients. In step 510, the DCT coefficients during reporting instance ‘n’ are compared with the DCT coefficients during reporting instance ‘n−1’ and for those DCT coefficients which have changed, the indices and DCT coefficients are complied. In step, 512 a subset of the changed DCT coefficients is chosen in accordance with predetermined selection metrics and this subset is transmitted from the receiver to the transmitter over a feedback channel in step 514. In step 516, the transmitter uses the DCT coefficients to regenerate CQI data for scheduling and AMC. Processing then proceeds to step 518 where the CQI reporting instance is incremented and the steps discussed above are repeated.

By now it should be appreciated that there has been provided a method and system for feeding back CQI by compressing the CQI feedback to reduce the average rate of CQI feedback. As described, the receiver estimates the channel state information for a transmission channel from a transmitting device by receiving one or more signals at the first receiving device. Using the channel state information, the receiver generates CQI information for the transmission channel by identifying a first CQI profile from a first codebook at the first receiving device that corresponds to the channel state information. For example, the CQI information may be a quantized CQI parameter, such as an index value from the first codebook that is associated with the first CQI profile that corresponds to the channel state information corresponding to the first CQI profile. In another example, a plurality of CQI profiles for a corresponding plurality of sub-carrier clusters (each having one or more sub-carriers) in a broadband channel may be identified and quantized into CQI indices. The CQI information is then compressed to remove redundancy from the CQI information, thereby generating compressed CQI information. A variety of compression techniques may be used. For example, the CQI information may be temporally compressed by sending CQI information for a predetermined CQI reporting instance over a feedback channel to the transmitting device only if a difference is detected between the CQI information for the predetermined CQI reporting instance and CQI information for a previous CQI reporting instance. Where there are a plurality of CQI profiles for a plurality of sub-carrier clusters, the plurality of CQI profiles may be compressed to remove temporal redundancy from the plurality of CQI profiles, thereby generating temporally compressed CQI information for each sub-carrier cluster. The temporally compressed CQI information may be sent over a feedback channel for only a first predetermined number F of the sub-carrier clusters whose CQI information has changed, where the predetermined number is either fixed or variable across a plurality of feedback interval. If the actual number C of sub-carrier clusters whose CQI information has changed is less than the first predetermined number F, then alternate information may be sent along with the temporally compressed CQI information, such that the alternate information is sent in place of the F-C sub-carrier clusters. However compressed, the compressed CQI information is fed back to the transmitting device where it is used to access a second codebook at the transmitting device to retrieve a second CQI profile that matches the first CQI profile and that is used to control transmission over the transmission channel from the transmitting device to the first receiving device.

The methods and systems for designing and using compressed CQI feedback in a limited feedback system as shown and described herein may be implemented in hardware or in software stored on a computer-readable medium and executed as a computer program on a general purpose or special purpose computer to perform certain tasks. For a hardware implementation, the elements used to perform various signal processing steps at the transmitter and/or at the receiver may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. In addition or in the alternative, a software implementation may be used, whereby some or all of the signal processing steps at each of the transmitter and receiver may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. It will be appreciated that the separation of functionality into modules is for illustrative purposes, and alternative embodiments may merge the functionality of multiple software modules into a single module or may impose an alternate decomposition of functionality of modules. In any software implementation, the software code may be executed by a processor or controller, with the code and any underlying or processed data being stored in any machine-readable or computer-readable storage medium, such as an on-board or external memory unit.

Although the described exemplary embodiments disclosed herein are directed to various communications systems and methods for using same, the present invention is not necessarily limited to the example embodiments illustrate herein. For example, various embodiments of a communication system disclosed herein may be implemented in connection with various proprietary or wireless communication standards, such as IEEE 802.16e, 3GPP-LTE, DVB and other systems. Thus, the particular embodiments disclosed above are illustrative only and should not be taken as limitations upon the present invention, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Accordingly, the foregoing description is not intended to limit the invention to the particular form set forth, but on the contrary, is intended to cover such alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims so that those skilled in the art should understand that they can make various changes, substitutions and alterations without departing from the spirit and scope of the invention in its broadest form.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. 

What is claimed is:
 1. An apparatus, comprising: one or more processing elements configured to: determine channel state information for wireless communications; determine a plurality of codebook indices corresponding to the determined channel state information; in response to determining that one or more of the codebook indices did not change relative to a previous transmission of channel state information, generate a compressed set of channel state information that omits the one or more of the codebook indices that did not change relative to the previous transmission of channel state information; and transmit the compressed set of channel state information.
 2. The apparatus of claim 1, wherein the channel state information includes channel matrix information for the wireless communications.
 3. The apparatus of claim 1, wherein the channel state information includes one or more signal to noise ratios.
 4. The apparatus of claim 1, wherein the compressed set of channel state information is usable to select an adaptive modulation and coding scheme for subsequent wireless communications.
 5. The apparatus of claim 1, wherein the compressed set of channel state information includes only transmission channel state information that meets predetermined selection metrics.
 6. The apparatus of claim 1, wherein the compressed set of channel state information includes one or more codebook indices.
 7. A method, comprising: receiving a compressed set of channel state information, wherein the compressed set of channel state information includes one or more codebook indices that changed relative to a previous transmission of channel state information and omits one or more codebook indices that did not change relative to the previous transmission; determining the omitted indices based on the previous transmission; and selecting an adaptive coding and modulation scheme based on the received codebook indices and determined omitted indices.
 8. The method of claim 7, wherein the channel state information includes channel matrix information.
 9. The method of claim 7, wherein the channel state information includes one or more signal to noise ratios.
 10. The method of claim 7, wherein the channel state information corresponds to multiple-input multiple-output (MIMO) communications.
 11. The method of claim 7, wherein the compressed set of channel state information includes only channel state information that meets predetermined selection metrics.
 12. The method of claim 7, wherein the compressed set of channel state information includes one or more codebook indices.
 13. An apparatus, comprising: at least one radio; a plurality of antennas coupled to the at least one radio; and one or more processing elements configured to: receive a compressed set of channel state information, wherein the compressed set of channel state information includes one or more codebook indices that changed relative to a previous transmission of channel state information and omits one or more codebook indices that did not change relative to the previous transmission; determine the omitted indices based on the previous transmission; and select an adaptive coding and modulation scheme based on the received codebook indices and determined omitted indices.
 14. The apparatus of claim 13, wherein the channel state information includes channel matrix information.
 15. The apparatus of claim 13, wherein the channel state information includes one or more signal to noise ratios.
 16. The apparatus of claim 13, wherein the channel state information is for a multiple-input multiple-output (MIMO) channel.
 17. The apparatus of claim 13, wherein the compressed set of channel state information includes only channel state information that meets predetermined selection metrics.
 18. The apparatus of claim 13, wherein the compressed set of channel state information includes one or more codebook indices. 